In /includes/functions.php the function page_header has as 2nd optional argument
...which will gather the online list data. However, this is needed on rather few pages. While the original development of phpBB3 might have had in mind that most of the default files are using this, time has shown that MODs with all their custom pages never use this data at all, yet they never call page_header( ... , FALSE ) to skip unneeded database activity.
This argument should be FALSE by default and update the 10 spots which would then need a TRUE as 2nd argument. A search thru all files of 3.0.11 gives me 9 spots which actually supply FALSE, while 10 spots use either TRUE or no argument.